The present invention relates to a method of mechanically producing diffraction gratings having grooves of varied line space, and particularly to a method of producing diffraction gratings in which the distance between grooves is changed continuously and greatly.
Nearly two hundred years have passed since the diffraction grating was first invented, and performance was improved strikingly. However, groove spacing of an equal distance have long been inherited irrespective of whether it is a plane diffraction grating or a concave diffraction grating.
In recent years, however, it has been proposed to arrange the grooves at irregular distances instead of arranging the grooves at equal invervals. That is, by arranging the grooves at irregular intervals on the concave diffraction grating, there have been proposed a variety of so-called aberration-corrected diffraction gratings having little or no aberration in the spectral image. So far, the plane diffraction grating did not have a light focusing property, and the cylindrical diffraction grating did not, either, have a light focusing property on a plane that includes a cylindrical axis. Recently, however, there have also been proposed plane diffraction gratings and cylindrical diffraction gratings having dispersing and focusing properties as a result of arranging the grooves at irregular intervals.
The aberration-corrected concave diffraction gratings have heretofore been produced by a method which is based upon the holography technology and by a mechanical method. However, the methods of producing aberration-corrected concave diffraction gratings are not applicable to the production of plane diffraction gratings having a focusing property or to the production of cylindrical diffraction gratings having focusing property.
According to the method based upon the holography technology, darkness and brightness of interference fringes by the recording laser beam are converted into rugged patterns of a photosensitive emulsion or a photosensitive resin, and a thin metal film is formed on the surface by vacuum evaporation or the like. In this method, limitation is imposed on the position of the source of recording laser beam and on the wavelength of the beam, making it difficult to impart a desired focusing property to the plane diffraction grating or the cylindrical diffraction grating. In particular, the diffraction gratings of this kind are usually used in a soft X-ray region through up to a vacuum ultraviolet region where strict limitations are required concerning the position of the source of recording laser beam and the wavelength of the beam. Therefore, it is virtually difficult to produce the diffraction gratings relying upon the holography technology.
Described below are a conventional method of mechanically producing the aberration-corrected concave diffraction grating and an apparatus therefor in conjunction with FIG. 1. Such a method has been taught, for example, in Japanese Patent Publication No. 33562/ 1982. In FIG. 1, the rotational force of a main motor 1 is transmitted to a worm reduction gear 3 via a belt 2. The rotational force is then transmitted to a tool reciprocating link device 4 which causes a tool carriage 5 to reciprocate, the tool carriage 5 being equipped with a groove-ruling tool. Rotational force of the worm reduction gear 3 is further transmitted to a feed screw 8 via a speed change gear 6 and a differential gear 7. A switch 9 is closed after every completion of a ruling, whereby a pulse motor 10 is turned by a predetermined angle, and the rotational force is transmitted to the feed screw 8 via the differential gear 7. A blank carriage 12 mounting a blank 11 is moved depending upon the rotational angle of the feed screw 8. While the tool is ruling the blank 11, the blank carriage 12 and the blank 11 are moved at a speed corresponding to a rotational speed determined by the speed change gear 6. During the period of from the completion of ruling by the tool to the next start of ruling, the blank carriage 12 and the blank 11 are moved by an amount consisting of a feeding amount (the amount of constant transference) corresponding to the rotational speed determined by the speed change gear 6 and a feeding amount (the amount of variable transference) corresponding to the number of pulses generated by a pulse generator 13, which is added thereto or is subtracted therefrom. Therefore, the grooves can be ruled at desired irregular intervals by controlling the number of pulses generated from the pulse generator 13 by a computer 14. The pulse generator 13 produces a predetermined number of pulses responsive to the instruction from the computer 14 after each opening and closing of the switch 9. The pulse motor 10 is turned by a rotational angle corresponding to the number of pulses generated from the pulse generator 13, and the rotational force is transmitted to the differential gear 7.
Here, the amount of variable transference is the product of a feeding amount of the blank 11 per unit pulse and the number of pulses. When the pulse motor 10 is used as a source for driving the blank 11 by unequal distances, the distance between grooves changes discretely strictly speaking. Usually, however, the blank 11 is fed by an amount of as small as 0.2 angstrom per unit pulse, so that the distance between grooves will virtually appear to change continuously. There has been realized a pulse motor which operates at a maximum drive frequency of about 10 kHz. If the pulse motor 10 is rotated at such a high speed, however, the rotational speed of a rotation transmission system subsequent to the differential gear 7 changes abruptly when the pulse motor 10 is started or stopped, and it becomes very difficult to properly feed the blank 11. To properly feed the blank 11, therefore, a maximum drive frequency for the pulse motor 10 must be limited to about 500 Hz. Further, the time in which the pulse motor 10 is allowed to turn is shorter than one-half the reciprocating period of the tool. Usually, the period for reciprocating the tool is about 6 seconds. Therefore, the time in which the pulse motor 10 is allowed to turn is about 3 seconds at the greatest. From the above fact, a maximum amount of variable transference is 0.2.times.10.sup.-4 .times.500.times.3=0.03 .mu.m.
Here, the object of the concave diffraction grating having groove spacing of irregular intervals is to correct the aberration. Therefore, the difference between the distance among grooves and the average distance of grooves is usually very small, i.e., the change of distance is very small, and the difference seldom exceeds 0.03 .mu.m at the greatest. Accordingly, the aberration-corrected concave diffraction grating can be sufficiently manufactured by the aforementioned conventional method. However, the object of the plane diffraction grating and the cylindrical diffraction grating having groove spacing of irregular intervals, are to exhibit the focusing property that had not hitherto been provided. In this case, therefore, the change of distance of grooves becomes inevitably large and a maximum value thereof greatly exceeds 0.03 .mu.m as will be mentioned later. Therefore, the conventional method tailored to producing the concave diffraction gratings is not virtually effective to produce the desired plane diffraction gratings or the cylindrical diffraction gratings. Even with the concave diffraction grating, it is difficult to perform the ruling by the conventional method if a maximum change of distance between grooves exceeds 0.03 .mu.m.
As described above, neither the method based upon the holography technology nor the mechanical method is effective for producing diffraction gratings having groove spacing of irregular intervals in which the distance between grooves vary greatly, such as the plane diffraction gratings and cylindrical diffraction gratings having focusing function.